The invention relates to optical signal monitoring, and more particularly to apparatus and methods for monitoring the wavelength and power of an optical communication signal.
Modem optical fiber communication systems have high bandwidth and low transmission loss. The bandwidth of an optical fiber determines how much information can be transmitted without losing data due to degradation in the optical signal. Many modem optical fiber communication systems use Wavelength Division Multiplexing (WDM).
In WDM communication systems, separate signals having different carrier wavelengths are transmitted simultaneously through a single optical fiber. The number of wavelengths simultaneously propagating in a fiber is proportional to the bandwidth of the communication system. Each wavelength bandwidth occupies a certain channel spacing in the communication system. The more closely spaced the carrier wavelengths, the more channels that can be propagated simultaneously. However, as the spacing between the wavelengths becomes smaller, the probability of cross talk between channels increases. This cross talk is undesirable because data from one channel interferes with data from another channel, thereby causing erroneous data to be propagated in the communication system and ultimately corrupting the data at the receiver.
In order to maximize the number of available channels in a WDM communication system, each laser source must generate an optical beam having a relatively stable wavelength. The lasers used for WDM transmitters generally emit light at a stable wavelength and the wavelength can be precisely controlled. However, many laser sources experience wavelength drift over time caused by temperature, aging, and modal instability. Wavelength drift can cause cross talk and result in a loss of data in WDM communication systems and, therefore, must be monitored and compensated.
Numerous apparatus and methods have been used to monitor the wavelength of optical signals in WDM communication systems. Some of these apparatus and methods split an input signal into two signals and filter one signal with a low-pass filter and the other signal with a high-pass filter. The filtered signals are directed to two closely spaced detectors. The electrical signals generated by the two detectors are then compared. By selecting the characteristics of the filters correctly, the wavelength of the optical signals can be precisely determined and monitored.
Other apparatus and methods used to monitor the wavelength of optical signals in WDM communication systems use a channel selector, such as a crystal grating or diffraction grating, and a wavemeter to monitor the optical signals. For example, in one prior art apparatus, an optical signal is first separated into channels by a channel selector and then propagated to a wavemeter that monitors the wavelength of the optical signal in each channel.
These prior art wavelength monitors are generally impractical for modem high capacity optical communication systems because they are complex and occupy relatively large volumes. In addition, these prior art wavelength monitors use differential detection methods to measure the wavelength of the communication signal, which can result in erroneous measurements. These differential detection methods require at least two photodetectors that each sample different portions of the waveguide mode.
Measuring different portions of the waveguide mode can lead to uncertainty in the measured wavelength due to modal instability. These uncertainties can result in erroneous measurements, which can result in incorrect compensation. If the waveguide is a single mode optical fiber, the mode is typically very stable. However, if the waveguide is multimode, there are modal instabilities under some conditions. There are several factors, which cause modal stability in the propagation media. These factors include the level of injection current, the condition of the facet coating, the efficiency and the operating temperature.
The present invention relates to wavelength and power monitors, which do not experience the disadvantages of differential detection and other prior art methods of monitoring wavelength. A principle discovery of the present invention is that an optical wavelength and power monitor can be constructed to monitor one portion of the mode of a single optical beam and can substantially simultaneously determine the wavelength and the optical power of a single optical beam.
Accordingly, the present invention features an optical beam monitor that includes a first detector positioned in the path of an optical beam. In one embodiment, the first detector comprises a semitransparent photodiode that transmits a portion of the optical beam. The first detector may include an anti-reflection coating that prevents a portion of the optical beam from reflecting off of the first detector. The first detector generates an electrical signal that is proportional to the optical power of the incident optical beam.
An optical filter, such as a Fabry-Perot filter or a thin film filter, is positioned in the path of the optical beam and passes a portion of the optical beam corresponding to a wavelength within the bandwidth of the optical filter. In one embodiment, the optical filter comprises a narrow band-pass filter. A substrate may be disposed between the first detector and the optical filter. In one embodiment, the substrate is formed of glass. In one embodiment, the substrate includes an anti-reflection coating on at least one end of the substrate to prevent reflections. In another embodiment, a glass wedge is disposed between the first detector and the optical filter in order to deflect any reflected beams away from the first photodiode, thereby reducing the detection of erroneous signals. The glass wedge may include an anti-reflection coating to prevent reflections.
A second detector is positioned in the path of the optical beam. The second detector generates a second electrical signal that is proportional to the optical power of the filtered optical beam. A processor is electrically coupled to the first and second detector and is used to generate a signal that characterizes the wavelength and power of the optical beam. This signal can be used to control the wavelength and power of the optical source that generates the optical beam.
The present invention also features an apparatus for monitoring the optical power and the wavelength of optical signals in a wavelength division multiplexed communication system. The apparatus includes a multi-wavelength laser that generates an optical beam. A first detector is positioned in.a path of the optical beam. The first detector generates a first electrical signal that is proportional to an optical power of the optical beam transmitting in the communication system. An optical filter is positioned in the path of the optical beam. The optical filter transmits a portion of the optical beam that corresponds to a channel of the communication system.
A second detector is positioned in the path of the optical beam. The second detector generates a second electrical signal that is proportional to an optical power of the filtered optical beam corresponding to the channel. A signal processor receives the first and second electrical signals. The signal processor generates at least one signal that corresponds to the wavelength and power of the optical beam transmitting in the communication system. This signal can be used to control the wavelength and power of the multi-wavelength laser.
The present invention also features a method for monitoring the wavelength and power of an optical beam. The method includes detecting an optical beam and generating a first electrical signal that corresponds to an optical power of the optical beam. A portion of the optical beam having a wavelength within a predetermined bandwidth is then detected. A second electrical signal is generated that corresponds to an optical power of the portion of the optical beam within the predetermined bandwidth. The first and second electrical signals are processed and a signal is generated that characterizes the optical beam. In one embodiment, the signal controls a source that generates the optical beam.
The above and other objects, features and advantages of the present invention and the manner of realizing them will become more apparent, and the invention itself will best be understood from a study of the following description and appended claims with reference to the attached drawings showing some of the numerous embodiments of the invention.